Field of the Invention
The present invention provides solid-state electrodes, including “indicator electrodes” (“IEs”) that utilize a redox-active analyte-insensitive material (“AIM”), and working electrodes (“WEs”) that utilize a redox-active analyte-sensitive material (“ASM”), for use in electrochemical analysis, including pH measurement, as well as methods for making and using the materials of the invention, and the electrodes, sensors, and devices comprising them. In one embodiment, an AIM is incorporated into an IE used in conjunction with a pseudo reference electrode (“PRE”) and an ASM that serves as a WE. The IE corrects drifts in the pH reading (or other analyte measurement) caused by changes in PRE potential, thereby improving the accuracy of the measurement and eliminating the need for calibration processes required for other types of analyte sensors. Accordingly, the invention relates to the fields of chemistry and electrochemistry in particular and to the field of analyte measurement, particularly pH measurement, and all the various fields where such measurements are made.
Description of Related Disclosures
Electrodes and analyte-sensing devices based on AIM and/or ASM electrochemistry have previously been described. See U.S. Pat. Nos. 4,857,167; 5,223,117; 7,901,555; 8,177,958; 8,197,650; 8,506,779; 8,562,797; 8,877,037; 8,956,519; 8,961,754, incorporated herein by reference. See also PCT Pub. Nos. 2010/026842, 2010/028726, 2011/045385, 2013/112767, and 2013/134582, and 2014/106066, each of which is incorporated herein by reference. Electrodes utilizing the AIM ferrocene are described in Lafitte et al. (2008), Electrochemistry Communications, 10(12), 1831-1834; Hickman et al. (1991), Science, 252(5006), 688-691; Robinson and Lawrence (2006), Electroanalysis, 18 (2006) 677-683, and Robinson and Lawrence (2008), Analytical Sciences 24. 339-343. While ferrocene hydrogels have been previously described in the art, these teachings and materials are limited to use as redox mediators for the purpose of transducing signals from a separate sensing element. Further, these references fail to teach or contemplate the physical properties of the ferrocene hydrogels discussed herein. Further, the present invention provides ferrocene hydrogel pH sensing elements that detect and report a reference redox signal.
Electrochemical sensors utilizing ASM/AIM and square wave voltammetry (SWV), sometimes called “voltammetric sensors,” have been hailed as providing an opportunity for “calibration-free” sensing (meaning that the sensor does not have to calibrated by the end user), especially for pH measurement. In these sensors, a pH-sensitive signal, generated by the WE, and a pH-insensitive signal, generated by the IE, are generated contemporaneously by a given analyte using SWV methods. If the analyte is hydronium ion (for pH measurement), the ASM WE changes potential in response to changes in hydronium ion concentration, whereas an ideal AIM for an IE in a pH meter generates a potential that is constant regardless of changes in the pH. Both of these potentials are measured relative to a reference electrode (RE) potential, as shown in FIG. 1. In this way, the difference between the ASM and AIM signals can be correlated to pH, or other analyte, and this difference ideally is independent of the absolute potential of the RE, which may be either a conventional reference electrode (“CRE”) or a PRE. Thus, for voltammetric sensors, changes in the RE have much less influence on the result than in conventional potentiometric pH sensors, which rely entirely on the stability and accuracy of the RE. Indeed, for voltammetric sensors, the simpler PRE (for example, a solid Ag/AgCl composition) may be used in place of a CRE (such as a Ag/AgCl or calomel CRE that is encased in a reference solution).
The need for calibration-free sensor technology remains, however, as none of the sensors referred to as “calibration-free” have proven to be so in practice and/or exhibit deficiencies in accuracy, signal strength, and/or durability. For example, previous self-calibrating sensors using ferrocene (Fc) as an internal reference electrode exhibited poor stability over time, and the Fc was sufficiently pH-insensitive only over a limited pH range. Any pH sensitivity decreases the precision of any required calibration and limits the reliability of the test result.
In terms of sensitivity, AIM electrodes (IEs) in the prior art based on Fc typically exhibit weaker, relatively noisy signals. In addition, the IEs and WEs of the prior art generate relatively weak and/or unstable signals, particularly under rigorous test conditions. Thus, a need remains for improved materials and methods for analyte sensing using an ASM-based WE and an AIM-based IE. The present invention meets this need.